102
6 Laboratory and Statistical Astronomy
hundred times smaller than that of the Sun. The formation of the chemical
elements in stars from hydrogen and helium is called stellar nucleosynthesis.
The relative amount of the various chemical elements in the Universe can be
understood in detail by this process, and I consider this to be one of the greatest
achievements of science.
In addition to our Galaxy, there are many other galaxies. Some look a bit
like ours, like the ones in Fig. 6.7, but they can also look very different. This
is because the relative contribution of the halo and the disk to the system may
be very different. For a good understanding of Kapteyn and his work, there is
no need to go into this in more detail here. 3
6.3 Parallaxes and Proper Motions
We saw above how Kapteyn showed that parallaxes of stars could be measured
from the timing of their passing the meridian. This success also had a downside,
because it was now clear that measuring parallaxes on a large scale was at least as
difficult as feared, and that the determination of distances of large quantities of
stars this way was impossible. Of course these measurements can also be done
photographically (with three exposures half a year apart) and Kapteyn even
suggested to do this as a matter of routine in the Carte du Ciel. But that, as we
have seen, was not accepted. The method is basically simple. Take a picture of
the sky when the shift of a star through the annual parallax is maximal. Leave
the plate for half a year until the shift is maximal in the opposite direction, and
make another exposure of the same star field on the same plate, but now with
the center of the plate shifted a little bit. Repeat that after another six months.
Assume that the faint stars on the plate are predominantly far away and have
little parallactic motion of their own and measure the shifts relative to those
background stars. The difference between the first and third recording then
show the annual proper motion. The second recording shows, compared to
the mean of the first and the third, the annual shift due to the parallax.
Kapteyn’s experience with the Carte du Ciel had, despite the decision not
to use his parallactic measuring method, also a positive outcome. At the great
congress in Paris in 1887 he had met Anders Severin Donner (1854–1938) of
the Helsingfors Observatory (Helsingfors is Swedish for Helsinki). They were
among the younger participants in the congress (see Figs. 5.10 and 6.8) and
between them a friendship developed that would last their whole lives. Anders
Donner was in favor of Kapteyn’s idea to measure parallaxes and proper motions
and with his telescope at Helsingfors—which he acquired for his participation
3 For more background see for example my course Structure and dynamics of galaxies [60].
6 Laboratory and Statistical Astronomy
hundred times smaller than that of the Sun. The formation of the chemical
elements in stars from hydrogen and helium is called stellar nucleosynthesis.
The relative amount of the various chemical elements in the Universe can be
understood in detail by this process, and I consider this to be one of the greatest
achievements of science.
In addition to our Galaxy, there are many other galaxies. Some look a bit
like ours, like the ones in Fig. 6.7, but they can also look very different. This
is because the relative contribution of the halo and the disk to the system may
be very different. For a good understanding of Kapteyn and his work, there is
no need to go into this in more detail here. 3
6.3 Parallaxes and Proper Motions
We saw above how Kapteyn showed that parallaxes of stars could be measured
from the timing of their passing the meridian. This success also had a downside,
because it was now clear that measuring parallaxes on a large scale was at least as
difficult as feared, and that the determination of distances of large quantities of
stars this way was impossible. Of course these measurements can also be done
photographically (with three exposures half a year apart) and Kapteyn even
suggested to do this as a matter of routine in the Carte du Ciel. But that, as we
have seen, was not accepted. The method is basically simple. Take a picture of
the sky when the shift of a star through the annual parallax is maximal. Leave
the plate for half a year until the shift is maximal in the opposite direction, and
make another exposure of the same star field on the same plate, but now with
the center of the plate shifted a little bit. Repeat that after another six months.
Assume that the faint stars on the plate are predominantly far away and have
little parallactic motion of their own and measure the shifts relative to those
background stars. The difference between the first and third recording then
show the annual proper motion. The second recording shows, compared to
the mean of the first and the third, the annual shift due to the parallax.
Kapteyn’s experience with the Carte du Ciel had, despite the decision not
to use his parallactic measuring method, also a positive outcome. At the great
congress in Paris in 1887 he had met Anders Severin Donner (1854–1938) of
the Helsingfors Observatory (Helsingfors is Swedish for Helsinki). They were
among the younger participants in the congress (see Figs. 5.10 and 6.8) and
between them a friendship developed that would last their whole lives. Anders
Donner was in favor of Kapteyn’s idea to measure parallaxes and proper motions
and with his telescope at Helsingfors—which he acquired for his participation
3 For more background see for example my course Structure and dynamics of galaxies [60].